Semiconductor laser device
FIELD OF THE INVENTION 
     The present invention relates to a semiconductor laser device and more particularly, to a laser that oscillate at a low threshold current. 
     BACKGROUND OF THE INVENTION 
     FIG. 2 shows a prior art semiconductor laser device having a bent active layer. In FIG. 2, reference numeral 1 designates a p type GaAs substrate. A p  type AlGaAs buffer layer 2 is disposed on p type GaAs substate 1. An N type GaAs blocking layer 3 is disposed on p type AlGaAs buffer layer 2. A p  type Al x  Ga 1-x  As lower cladding layer 4 is disposed on n type GaAs blocking layer 3 and p type buffer layer 2. An undoped Al y  Ga 1-y  As active layer 5 is disposed on lower cladding layer 4. An N type Al x&#39;  Ga 1-x&#39;  As upper cladding layer 6 is disposed on active layer 5. An N type GaAs contact layer 7 is disposed on upper cladding layer 6. Reference numeral 8 designates effective current flow and reference numeral 9 designates noneffective current flow. Herein, x and x&#39; are larger than y. 
     The device will operates as follows. 
     The current injected from the p type substrate 1 flows through the p type AlGaAs buffer layer 2 and further through the p type Al x  Ga 1-x  As lower cladding layer 4, the updoped Al y  Ga 1-y  As active layer 5, the n type Al x&#39;Ga   1-x&#39;  As upper cladding layer 6, and n type GaAs contact layer 7. Herein, current is not injected except at a bent portion of the undoped Al y  Ga 1-y  As active layer 5 because n type GaAs current blocking layers 3 are located between the p type AlGaAs buffer layer 2 and the p type Al x  Ga 1-x  As lower cladding layer 4. As a result, an effective current 8 is injected into the neighborhood of the bent portion of undoped Al y  Ga 1-y  As active layer 5, producing laser oscillation. 
     In the prior art semiconductor laser device having a bent active layer, a portion of the current injected from the p type GaAs substrate 1 is not injected into the bent portion of undoped Al y  Ga 1-y  As active layer 5 but is injected into the flat portion at the both sides of the bent portion as noneffective current 9, increasing the threshold current. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a semiconductor laser device oscillating at a low threshold current. 
     Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modification within the spirit and the scope of the invention will become apparent to those skilled in the art from this detailed description. 
     According to an aspect of the present invention, impurities are diffused intermediate the lower or upper cladding layers not reaching the bent portion of active layer, thereby producing a region reverse in conductivity type to the conductivity of the lower or upper cladding layer. The pn junctions in the cladding layer produced by diffusing impurities obstructs noneffective current, thereby reducing the threshold current. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view showing a semiconductor laser device according to an embodiment of the present invention; and 
     FIG. 2 is a cross-sectional view showing a semiconductor laser device according to the prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiments of the present invention will be described in detail with reference to the drawings. 
     FIG. 1 shows a semiconductor laser device according to a first embodiment of the present invention. In FIG. 1, the same reference numeral designate the same or corresponding elements shown in FIG. 2. Reference numeral 10 designates a Si diffusion region, and reference numeral 11 designates a pn junction having a high built-in potential. 
     The laser device will operate as follows. 
     The flow of current is almost the same as in the prior art device but is different as described below: 
     When current is injected into the undoped Al y  Ga 1-y  As active layer 5 from the p type Al x  Ga 1-x  As lower cladding layer 4, noneffective current 9 is not injected into the flat portion the side of the bent portion as in the prior art device. On the contrary, in the present embodiment, only effective current 8 is injected. In the present invention, Si is diffused into the surface opposite the substrate to produce a Si diffusion region 10 which is n type. Region 10 extends to a position within the p type Al x  Ga 1-x  As lower cladding layer 4, and a pn junction 11 having a high built-in potential is produced within the p type Al x  Ga 1-x  As lower cladding layer 4. This potential barrier is higher than the potential barrier of the p-i-n junction produced at the bent portion between the lower cladding layer 4, the undoped Al y  Ga 1-y  As active layer 5, and the upper cladding layer 6 because x&gt;y. Therefore, current (a noneffective current 9) is not likely to be injected into the flat portion at the side of the bent portion. Thus, a semiconductor laser device of low threshold current having reduced noneffective current 9 is realized. 
     While in the above-illustrated embodiment, a semiconductor laser device has an active layer which is bent in concave configuration toward the substrate, a semiconductor laser device having a curved active layer may be employed. Furthermore, a semiconductor laser device having an active layer which is bent in convex configuration or curved away from the substrate may be employed by diffusing dopant impurities from the substrate side into the upper cladding layer. Furthermore, dopants other than Si may be used for the diffusion. Furthermore, while in the above-illustrated laser device the cladding layers and active layer comprise AlGaAs series material, a semiconductor laser device comprising AlGaInP series or GaInAsP series materials may be employed. 
     As is evident from the foregoing description, according to the present invention, impurities producing a region of reverse conductivity type from that of the lower or upper cladding layer are diffused the lower or upper cladding layer from opposite the substrate side or the substrate side, whereby noneffective current flow is obstructed by the pin junction produced by diffusion the impurities and only effective current flows through the bent or curved portion of the active region, thereby enabling oscillation at a low threshold current.